1. Field of the Invention
The present invention relates to thermal analytical techniques for determining the composition, phase, structure, or other properties of a sample of material.
2. Background of the Invention
Thermal analysis techniques generally comprise measuring a physical parameter as a function of the temperature of a material. Whenever a material undergoes a chemical transformation, a physical transformation, a phase change or another transition which affects the physical parameter being measured, the changes in that physical parameter may be interpreted to analyze the composition, structure, or thermal stability of the sample.
Traditional thermal analysis techniques can be improved by modulating the temperature of the sample and reference. Modulated differential thermal analysis techniques are described in the grandparent application, U.S. Pat. No. 5,224,775 to Reading et al. Modulated differential techniques improve the interpretation of differential scanning calorimetry ("DSC"; explained in more detail below) data by adding a sinusoidal temperature modulation to a linear DSC temperature ramp and deconvoluting the resultant heat flow signal into rapidly reversible and non-rapidly reversible components. In modulated DSC, just as in conventional DSC, the sample and reference materials are exposed to identical temperature programs.
DSC is a thermal analysis technique which measures the heat flows to/from a sample material and an inert reference material, as the sample and reference are exposed to the same controlled temperature. The difference in the heat flow measured for the sample and that measured for the reference is recorded, from which physical parameters of the sample are determined. See, e.g., W. W. Wendlandt, Thermal Methods of Analysis, 193-201 (1974).
Differential scanning calorimeters fall into two broad classes of instruments: heat flux DSC and power compensated DSC. Heat flux DSCs measure the dynamic temperature difference between a sample material and a reference material. Because the dynamic temperature difference is proportional to the total heat flow to/from the sample, the total heat flow to/from the sample is obtained from the dynamic temperature difference. See, e.g., U.S. Pat. No. 4,095,453 to Woo.
Power compensated differential scanning calorimeters control the total flow of heat to the sample material and reference material separately. The total flow of heat is controlled so as to maintain the temperature of the sample material at the temperature of the reference material during physical transformations in the sample material. The total heat flow to/from the sample material is calculated from the difference between the power supplied to the sample material and the power supplied to the reference material. See, e.g., U.S. Pat. No. 3,263,484 to Watson et al.
In traditional differential scanning calorimetry, the sample material and the reference material are simultaneously subjected to the regulated temperature environment. However, this is not essential to the operation of the calorimeter. Differential scanning calorimetry may be performed sequentially, by subjecting the sample material and the reference material to consecutive measurements, storing the results, and subsequently calculating the total heat flow to/from the sample. U.S. Pat. No. 4,848,921 to Kunze describes this technique in power compensation calorimeters. In principle, heat flux calorimeters could also measure the heat flow to/from the sample material and the reference material sequentially.
Differential Thermal Analysis ("DTA") is a thermal analysis technique similar to DSC, wherein the temperatures and heat flow associated with transitions in materials are measured as a function of temperature. However, unlike DSC, DTA results are semi-quantitative. DTA is generally carried out at higher temperatures than DSC.
Another common thermal analysis technique is AC calorimetry. AC calorimetry is a thermal analysis technique which measures the heat capacity or thermal diffusivity associated with chemical or physical transitions in materials as a function of time and temperature. In AC calorimetry the heat energy flow to/from the sample is controlled. The resulting temperature change of the sample is measured and recorded, from which the physical parameters of the sample are determined.
One AC calorimetry technique measures the heat diffusion through a material adiabatically. In this technique, a modulated heat source is applied to one surface of a thin flat sample of material, while the resultant temperature oscillations on the opposite surface are measured and recorded. The modulated heat source is typically chopped light, laser flash, or direct Joule-heating from a resistive heating element or furnace. A lock-in amplifier is typically used to detect the AC temperature being measured. See I. Hatta and A. J. Ikushima, Japanese Journal of Applied Physics, vol. 20, pp. 1995-2011 (1981); and S. Ikeda and Y. Ishikawa, Japanese Journal of Applied Physics, vol. 18, pp. 1367-1372 (1979). This technique does not measure the total heat flow to/from a sample, but instead measures the response at the rear face of a sample to an AC heat input at the front face of the sample. See Hatta et al., referenced above, FIG. 1a and pp. 1996-97. The heat capacity of the sample can then be calculated from the amplitudes of the sine components for the measured temperature oscillation and input heat signal. Id.
Another AC calorimetry technique measures the heat diffusion into a material nonadiabatically. In this technique a modulated heat source is applied to one surface of a sample, and the resultant temperature change at the point of heat application is measured and recorded. The modulated heat source is typically direct Joule-heating from a resistive heating element or furnace. The major benefits of this technique, as compared to adiabatic AC calorimetry, are reduced constraints on sample geometry and modulation frequency (which is limited only by the frequency of the heat source modulations. See N. O. Birge, Physical Review B, vol. 34, pp. 1631-1642 (1986); and D. H. Jung et al., Meas. Sci. Technol., vol. 3, pp. 475-476 (1992).
High resolution techniques are described in U.S. Pat. No. 5,165,792 to Crowe et al., which is incorporated by reference herein. High resolution techniques seek to improve the resolution of changes in a characterizing physical parameter by controlling the rate of sample heating during transitions as a function of the rate of change of the physical parameter. When non-differential thermal analysis techniques are used, the high resolution techniques are effective in improving resolution for many transitions. However, they usually reduce the sensitivity of transitions when applied to differential thermal analysis techniques. This is because, for most differential thermal analysis techniques, the magnitude of the differential physical parameter is a direct function of the heating rate. Reducing the heating rate during transitions causes the differential signal to change, which may alter or obscure the true differential signal resulting from the transition event. This obscuring of the physical parameter can reduce the utility of the high resolution techniques when applied to conventional differential thermal analysis techniques.
It is often advantageous to combine two or more characterizing physical parameters to more precisely characterize a material. However, when conventional thermal analysis techniques require multiple samples or multiple measurements, the accuracy of the results are affected by run-to-run and/or sample-to-sample variations. Furthermore, the additional experimental steps and apparatus required to perform separate measurements reduces laboratory productivity compared to a simultaneous measurement.
Conventional thermal analysis techniques, including DSC, DTA and AC calorimetry, are limited in their ability to separate non-reversible transitions caused by enthalpic processes (chemical or physical) from reversible transitions such as changes in the heat capacity of the sample. This is because the reversible and non-reversible processes often occur simultaneously, or severely overlapped in time and/or temperature.
In addition, both conventional and high resolution thermal analysis techniques cannot distinguish between rapidly reversible and non-rapidly reversible transitions within a single heating or cooling scan of the sample.